1. Field of the Invention
The invention relates to an improved well safety valve for use in controlling well fluids conducted through the well tubing bore. More particularly, the safety valve possesses a fail-closed system for dual pressure fluid operation.
2. The Prior Art
Surface controlled subsurface safety valves have been used to control flow in a well at a subsurface location for some time. It is well known to provide a well safety valve to control the fluid flow in a well conduit by means of a valve which is movable between an open and closed position and in which means are provided to move the valve toward the closed position and control pressure acts to move the valve toward an open position. If the means for moving the valve to the closed position includes a pressurized chamber, the pressure in the pressurized chamber is normally enclosed by seals. Usually, in such subsurface safety valves a spring or other resilient urging means is used to move the valve to the closed position. The balance fluid pressure is used to overcome the hydrostatic pressure present in the column of hydraulic fluid which operates the means responsive to the control fluid pressure in opening the safety valve.
The entry ports through which control and balance fluid enters the landing nipple housing the safety valve are positioned in such a manner that they are essentially isolated one from the other and each pressure zone must withstand tubing bore pressure by operation of seals surrounding such entry ports for the pressure chambers. Thus, in the event of seal failure in the area of the control pressure chamber tubing bore pressure would probably enter the control pressure chamber causing the valve to fail in the open position. Such occurrence would undoubtedly cause a dangerous situation since there would be no safety valve system then closable to prevent escape of tubing pressure or fluids to the surface of the well.
Consequently, there has been a search to develop a surface controlled subsurface safety valve having a seal arrangement that would provide a fail-closed safety system.